1. Field of Invention
The present invention relates to a solid state relay; and more particularly, to a solid state relay for the selective interruption of high amperage direct current.
While the invention is subject to a wide range of applications, it is especially suited for use in a high amperage solid state circuit breaker used in DC systems, and is particularly described in that connection.
2. Description of Related Art
Conventional switches that utilize mechanical contactors are able to pass high current efficiently because the entire current path consists of metal conductors. However, opening the mechanical contact switch requires the physical movement of mechanical parts, which requires a switching time in the order of milliseconds. During this time, fault currents can build to high levels. In AC systems this is not a serious problem, because all AC current passes through zero on each cycle, at which time arc currents can be extinguished. In DC systems, however, there is no natural current reversal, and the high currents must be forcibly interrupted. The severe stresses and high energy dissipation of the arc can result in contact failure, which is a significant reliability problem.
In order to overcome the disadvantages of devices using only mechanical contactors to break a high amperage direct current, solid state relays have been proposed. In circuit breakers a solid state relay is able to interrupt fault currents with switching times in the order of a microsecond, for example, without arcing. Because of the speed of solid state switching, the fault currents may be limited to much lower levels than those seen in mechanical switches. Unfortunately, steady state currents must pass through a semiconductor junction, with the attendant forward voltage drop and power loss. This typically requires supplemental cooling for the solid state relay, either by forced air, conduction, or liquid cooling.
For applications where the direct current to be passed continuously is typically in the order of several hundred amperes, the total amount of power loss of semiconductor devices is limited by the volume available for cooling fins to transfer waste heat to the ambient air, and by the specified ambient and semiconductor junction temperatures. The maximum permissible forward voltage drop for a solid state switching relay, is calculated from these considerations, and is typically only a small fraction of a volt. This eliminates from consideration devices such as bipolar transistors and gate turn-off thyristors, for example, with a minimum voltage drop in excess of one volt regardless of the current passing through them. For such applications, it has been proposed to build a solid state relay with low conduction loss requiring the use of many metal oxide semiconductor field effect transistors (MOSFET's) in parallel. This requires that enough semiconductor devices be operated in parallel to provide a total resistance below one milliohm, for example. Thus, several hundred of these devices must often be used to meet such requirements. Such a proposed arrangement would provide faster operation, programmable trip characteristics, and potentially more reliable operation than mechanical circuit breakers because of the elimination of arcing due to the interruption of large DC fault currents. However, the disadvantages of this proposed arrangement are physical size, material costs, and production yield. It is well known that the more devices that are used in a single assembly, the less chance there is that they will all work properly the first time. When hundreds of devices are used in a single assembly, the chance that they will all work approaches zero. The large footprint area required for several hundred of the MOSFET devices in a single package, and the cost of conventional semiconductor packages or power hybrids renders their use impractical. Additionally, the cost of the heat sink itself is significantly increased by the need to include fins to increase thermal efficiency by breaking boundary layers. Further, the need to provide efficient electrical and thermal connections to the MOSFET devices results in complicated three-dimensional construction of bus bars and heat sinks which require extensive hand labor and assembly.
In light of the foregoing, there is a need for a solid state relay, and a circuit breaker incorporating a solid state relay, that is capable of operating at several hundred amperes with low resistance to continuous current, operates substantially faster and more reliably than a mechanical switch contactor, requires no cooling other than free air convection, does not occupy a large area, and is relatively easy to manufacture and assemble.